1. Field of the Invention
This invention relates to a linear motor device, and more particularly to a linear motor device used as driving means or the like for driving the optical pick-up unit of an optical type recording-reproducing apparatus.
2. Related Background Art
In recent years, the tendency of recording-reproducing apparatuses to have a large capacity has increased and attention has been paid to optical disc memories as high-density recording mediums therefor. Now, to record information in such an optical disc memory or read out the recorded information, it is necessary to drive a carriage carrying thereon an optical pick-up movable along the surface of the optical disc, and a linear motor of the voice coil type has been used as driving means therefor.
FIGS. 9 and 10 of the accompanying drawings shows an example of an optical pick-up driving apparatus using such a linear motor. In these figures, the reference numeral 1 designates a disc supported by a hub 2 and rotatively driven by a spindle shaft 3, and relative to such a disc 1, an optical pick-up 5 provided with an objective lens 4 is carried on a head carriage 6, which is movable in the direction of arrow along shafts 8 on a pedestal 7. The reference numeral 10 denotes linear motors for driving the head carriage 6.
The linear motors 10 are provided at the opposite sides of the shafts 8, and as shown in FIG. 11 of the accompanying drawings, each of the linear motors is comprised of a center yoke 11, a side yoke 12, a magnet 13 fixed to the side yoke 12, a relay yoke 14 for coupling the center yoke 11 and the side yoke 12 together at their opposite ends, a coil bobbin 15 movable along the center yoke 11, and a coil 16 wound on the bobbin 15. Mounting portions 15A of the bobbin 15 are fixed to the opposite end portions of the head carriage 6.
So, in the linear motor 10 thus constructed, by supplying an electric current to the coil 16 through a magnetic circuit constituted by the center yoke 11, the relay yoke 14, the side yoke 12 and the magnet 13, the coil 16 can be moved with the bobbin 15 along the center yoke 11, thereby moving the head carriage 6.
Now, in such a prior-art linear motor 10, as shown in FIG. 12 of the accompanying drawings, it is necessary to keep a gap L.sub.G between the center yoke 11 and the inner surface of the coil bobbin 15 which is opposed to the center yoke 11 in order to make the coil bobbin 15 movable along the center yoke 11. This gap L.sub.G should desirably be at least 0.5 mm from a consideration of the assembly error of the linear motor 10 or dynamic factors such as the deformation of the bobbin due to the heat generation resulting from the electrical energization of the coil and the deformation due to the inertia force caused, by the generation of an electromagnetic force large enough to move the head carriage unit.
Also, when the thickness of the coil-wound portion 15B of the coil bobbin 15 is L.sub.B and the height of the rib portion 15c at the opposite ends thereof is L.sub.H, the thickness L.sub.B should desirably be 0.5 mm or more from the viewpoints of strength and shaping factors, and the height L.sub.H of the rib portion should desirably be greater than the winding thickness L.sub.C of the coil 16.
Accordingly, the interval L kept between the magnet 13 and the center yoke 11 is EQU L=L.sub.B +L.sub.H +2L.sub.G. (1)
So, the interval L need be the rib height L.sub.H prescribed by the coil winding thickness L.sub.C, plus at least 1.5 mm.